You may well be right. 

It appears that it is connected to being awarded a secret achievement.

A current work around has been posted several times already on the forum.

Other possible solutions.

#Don't attempt the achievements whilst there is a problem.

#Make sure you take off a piece of equipment, before logging off.

I believe it  works but haven't been able to confirm yet

KROW said:

You may well be right. 

It appears that it is connected to being awarded a secret achievement.

A current work around has been posted several times already on the forum.

Other possible solutions.

#Don't attempt the achievements whilst there is a problem.

#Make sure you take off a piece of equipment, before logging off.

I believe it  works but haven't been able to confirm yet

taking off the equipment doesn't make any difference. i run 4 accounts ( and yes I spent $ on all 4 of them), all the same invader techs, same town skins. but the heros have different energy levels ranging from 200 to 3,300. the lowest lv. hero receiving the most and the highest one only receiving 200 less. they range between hero lvs 50 and 54. 50 had 4,300 this morning, hero on acct.2 is lv51  and it has same techs as the others but it's max energy reserve  was only 1000, both them accounts are wearing nothing for equipment at the moment. and this morning, account 1, the one that had 4,3k energy reserve max only has 3,3k max now. It seems like when I log off and re-log back in later i am not sure how much is going to be missing, it appears to change with every log in? I am not really sure, all I know is account 1 had 4,300 something this morning on full energy charge, I just logged in again and it is now showing 3,345 for max energy. Something is seriously wrong with hero energy in this game!

Also, I just came back to the game last week so that means I still have the old town skins and at the moment all accounts are wearing the Bastion of the beast skin which is supposed to give another 1000 units of energy, my account #2 is not receiving anything for bonus. not from techs and not from the old town skin.

The reason I thought having a piece missing was a solution, because  two accounts got an award about the same time, one had the energy problem the other didn't, subsequently I too found that the  if the hero had a piece of equipment missing, it didn't help.

KROW said:

The reason I thought having a piece missing was a solution, because  two accounts got an award about the same time, one had the energy problem the other didn't, subsequently I too found that the  if the hero had a piece of equipment missing, it didn't help.

So either I was mistaken , something changed , or as you wrote the change is variable. 

Last night I logged in and the hero was still broken as I call it at 1k. I hit one invader and noticed it said 12 hours to recharge and i thought wtf? 12 hours to recharge for 1k? so I looked at it's energy lv. and it went back to 4225, so I logged off and when I came back this morning the hero didn't recharge at all and it went from 4225 back down to 1k again and was stuck at 200 and didnt recharge at all.

Also idk if its because the energy lv. is broken but it seems like the hero is not getting her tech bonus, the way it hits invaders is like a low level hero. and i have every invader tech maxed out to lv10 all the way up to lv6. but it looks and act's like a lv5 hero. which btw it's a lv51. and when I added energy drinks today to do a few killings for the invader achievement it seems like it was not getting full credit for the 2k drinks. I would add a drink and it's like i only got 2  hits and thats it, the energy bar showed I had lot of energy but the game would only allow me 2 hits on a lv1 invader. so im not going to be using the drinks anymore til they get this fixed.

The reason I thought having a piece missing was a solution, because  two accounts got an award about the same time, one had the energy problem the other didn't, subsequently I too found that the  if the hero had a piece of equipment missing, it didn't help.

So either I was mistaken , something changed , or as you wrote the change is variable. 

At the moment maximum hero level is like driving a car and having to check frequently, if all 4 wheels are still on.

Dear Jarls,

We are sorry for any inconvenience caused. We are already aware of the issue and are working to resolve it. We assure you that our technicians are doing their best. Also if you have this issue you can try to take off and equip any Hero’s equipment.

We hope for your understanding. Have a nice day!