It's a possibility and it even makes sense
if you upgrade only your main weapon initially you will see an improvement but passing a certain level of upgrade makes the dungeons more difficult making you think it's the rest of your equipment
Kinda reminding me to this video
I have never noticed it on mobile games, or maybe i'm just dumb to not notice them. Sadly, it's possible someone taking advantage to make you pay more. I'm glad I never paid anything to mobile games in my entire life, instead I pay for the most important stuff... the food XD