I'm using the OpenType version of an alleged "heavy" font, called Osake (link to dafont).

I've done some typesetting and animation through use of Aegisub (using libass for subtitle previews).
The typesetting utilizes a couple of vsfiltermod tags like \vc and \va so to my knowledge I have to use TextSubMod to load my .ass file anywhere, whether it's directly in VirtualDub or as an .avs file for use with ffmpeg.

However, certain glyphs randomly don't display in the output when the .ass file is loaded, in most cases it's the U character and the first time I tried it was the O character.
I have tried reinstalling the font as both TrueType and OpenType but there's no apparent difference.
The Windows font previewer thing displays all glyphs as they should, including capital U and O, and the Aegisub preview displays everything correctly.

I'm thinking there may be a memory issue with either VSFilterMod (version 5.2.5 if I'm not mistaken) or AviSynth+ 3.7.0 since the font is heavy and uncompressed may exceed a certain memory threshold, but I haven't had much luck in my googlings for that.

Is there anything else I can try to rectify this issue?