In Belathis it happened at the boss fight which makes the whole instance mode unplayable for those lesser geared (and the main quest line runs through Belathis). Maybe the trash mob spawns at the beginning of the instance not giving any XP/TP are part of a learning process on Runewaker's side back from what happened in ToSH, but that is also speculation on my end.
The ToSH situation would have probably been officially declared an exploit if you would gain the amount of XP that hardmode mobs give while simply killing easy mode mobs. But since that is not the case, and merely the mother bugs in all three mods spawning the same ID of the child bugs, the one meant for hardmode (although by looking through some data quickly on the romwelten page there seem to be rather 5-6 different little mechanic bugs instead of the expected 3 ...) , you are getting the correct reward (xp for a hardmode mob) for killing the correct mob (a hardmode one), regardless where it came from.
Yes, super-strictly speaking the spawn should output a different version of the mob. But after looking at the situation, it was declared as ok to do in the past (that does not mean we put big signs everywhere that say "Hey, powerlevel here, yours truly, GF/GMs") , and I don't have any other information about it today, so ... fine with me.
You can still go and clear out any non bugmother mobs in that area on hardmode (or all the way till first boss) and then simply do the same thing. The difficulty is roughly the same today and a hit or two from a hardmode mother bug does not make a difference when there are endless amounts of little ones on the screen, if enough hit, you die. Unless the level of the farmer is so high already that he will dodge everything due to the level difference, but then it doesn't matter if he does not get hit by an easy mother bug or a hard one.
In the end: You can do it. What you of course shouldn't do is bug/exploit the honor group on purpose to level higher... - if you start leveling at 1 and just keep it as long it runs, no one minds. But if I see a level 70 with a "2h59 left" honor group buff ... that I do mind.
(Did I just agree with Borella on this? Really? ....
)