Not ruling out tank size as a possible issue, but I've done this several times in my 10G at the original 4 tbsp/10G dose without problems.
Asking if 50% water changes are routine is a great idea. This could drastically alter parameters if the tank normally gets small changes, or just top-offs.
Another thought just crossed my mind. H2O2 becomes a more powerful oxidant at low pH. When I want to avoid bleach, I sometimes spritz hardscape outside of the tank with vinegar, then H2O2, and it burns through stuff that H2O2 alone won't. On micron media it also removes stains and restores flow much better than bleach or H2O2. I have no idea if this is significant at any pH that one might encounter in an aquarium, though.
Life happens. On semi-hiatus, checking in infrequently.