Hard to imagine... are you sure that the computer with AC 15 has Stylus BT installed?
I just took the Stylus BT font file from an XP machine and installed it on my Mac and AC 15 has no problem with it. This old 1995 Bitstream font, distributed by Microsoft, might have other issues causing the problem?
When you say "substituted", if you select a block of text that WAS Stylus BT, does the font show up in square brackets as [Stylus BT] - and then the appearance is different? The square brackets mean that AC cannot find a font with that name. Moving files from Windows to Mac often results in this issue, as one or the other platform appends Western or Roman (etc) making the font name not match exactly - even though the font is installed. My experience is that moving from Mac to Windows is never a problem with fonts, but the other direction often has name matching issues.
Karl
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
One of the forum moderators