Looks like I arrived late at the party.
In any way, you did one of those steps wrong, because I can go into the credits without any issues using both the glitch method to finish the game in less than 3 minutes and the proper way by loading the ROM-> Enter Pyramid -> Defeat Ganon.
I tested this with the ROM in which I HEX edited the portions of the monologue for the Triforce and it's all working correctly.
If you are somehow getting glitches in the credits then perhaps you did not delete the 52 bytes I deleted after the data bank for the monologue is over.
I counted exactly how many bytes I added for the Triforce message, and then simply deleted the exact number of bytes from the closest free space of the ROM to compensate for that and to not screw up any other addresses in the ROM.
I tested it rigorously, the whole credits and also tried testing each monologue that comes after the Triforce one, and they all showed up fine.
So perhaps what @vortex says is true, maybe you corrupted the ROM in some form or another.
The HEX edited ROM with the fixed monologue was NOT opened up again with Hyrule Magic, so maybe it is screwing up some parts of the ROM.
Sucks that the project's source is not available to fix these kind of problems...