• When hammering stone with an activated (advanced) jackhammer (3x3 mode) the stone is broken, but +8 is added to the amount of the jackhammer (so after breaking once you have 9 jackhammers in the stack; can also exceed 64 later)
• When the jackhammer is at 0 power and another block is broken the jackhammer vanishes

Tried:
Tech Reborn 3.5.4 build 122
Tech Reborn 3.5.3 build 115
on MC 1.16.2

I am having the same issue with the Industrial Jackhammer in 5x5 mode, though it seemed to be adding even more than 8 at a time. Didn't try it on 3x3 mode when after it started glitching dozens of Industrial Jackhammers.

Indeed I have! I couln't reproduce it in my test instance with just TR either.
After updating Allure from 1.2.6 to 1.2.7 the jackhammer doesn't duplicate and destroys itself anymore.

Sorry for reporting and thank you for the tip! Definitely was someting on Allures end. The last changelog even says "Fixed bug with tools being duplicated after use!"

Do you have Allure mod installed?

I can't reproduce it in dev env.

Just broke 8 blocks and haven't got any additional jackhammers