For variable fonts; cv13 only changes б into Serbian Cyrillic form; but г, д, и, п, and т don't get changed into Serbian Cyrillic forms.

Reproduce el problema con Monaspace Argon Var.ttf en Fontdrop usando slant 0 y -11, con cv13 habilitado, y compárelo con MonaspaceArgon-ExtraLightItalic.otf. Inspeccione cómo la fuente variable gestiona los seis caracteres cirílicos serbios б, г, д, и, п y т; se considera completado cuando cv13 produce los cambios de glifo esperados en la fuente variable, coincidiendo con el comportamiento de la fuente estática en cursiva.

Disclaimer: I don't speak languages that use Cyrillic characters so my knowledge of the script is fairly limited.

I was reading about the difference of Cyrillic characters between those in Russia, Bulgaria (available within the opentype feature cv12 in Monaspace fonts) and Serbia (available within the opentype feature cv13 in Monaspace fonts).
See also: https://en.wikipedia.org/wiki/Cyrillic_alphabets#/media/File:Cyrillic_alternates.svg

Afterwards I looked into implementation of this on other fonts, it seems that Cascadia Code/Cascadia Mono implement both Bulgarian and Serbian Cyrillic glyphs.

• Bulgarian Cyrillic is available within the opentype feature locl. (which is harder to utilize than cvxx / ssxx so I think that Monaspace's implementation is better than that of Cascadia in this regard)
• Serbian Cyrillic is available for the character б, г, п and т тwithin the opentype feature ss03, but only for the italic font files. (No upright Serbian Cyrillic glyphs)

Back to Monaspace fonts, after I inspect some Monaspace font files with Fontdrop, it appears that cv13 is supposed to be able to alter up to six characters: б, г, д, и, п, and т. Where only the first character (б) would be changed into in the Serbian Cyrillic glyph in the upright fonts, and all of these 6 characters would be changed into in the Serbian Cyrillic glyph in the italic fonts. Although all fonts have no problem changing the first character (б) into the Serbian Cyrillic glyph. I couldn't get the variable font files to change the remaining 5 characters into the Serbian Cyrillic glyph at all. Which is strange since all italic static font files I've tested can change all 6 characters into the Serbian Cyrillic glyph with no problem.

Gallery:
Image 1, viewing Monaspace Argon Var.ttf with Fontdrop with slant=0 (upright) and cv13 disabled

Image 2, viewing Monaspace Argon Var.ttf with Fontdrop with slant=-11 (italic) and cv13 disabled

Image 3, viewing Monaspace Argon Var.ttf with Fontdrop with slant=0 (upright) and cv13 enabled

Image 4, viewing Monaspace Argon Var.ttf with Fontdrop with slant=-11 (italic) and cv13 enabled

Image 5, viewing MonaspaceArgon-ExtraLight.otf with Fontdrop with cv13 disabled

Image 6, viewing MonaspaceArgon-ExtraLightItalic.otf with Fontdrop with cv13 disabled

Image 7, viewing MonaspaceArgon-ExtraLight.otf with Fontdrop with cv13 enabled

Image 8, viewing MonaspaceArgon-ExtraLightItalic.otf with Fontdrop with cv13 enabled

As it can be seen here, behavior of variable fonts in image 4 is incorrect and, to the best of my understanding, should be like in image 8.

Closing notes:

1. I haven't checked every font files as there are 1485 Monaspace font files in total. (As of current version: 1.4)
2. I haven't seriously checked on the Bulgarian Cyrillic glyphs. Though everything about them seems normal for the time being.
3. The text shown in the above screenshots is:

ШИРОКАЯ ЭЛЕКТРИФИКАЦИЯ ЮЖНЫХ ГУБЕРНИЙ ДАСТ МОЩНЫЙ ТОЛЧОК ПОДЪЁМУ СЕЛЬСКОГО ХОЗЯЙСТВА.
Б Г Д И П Т
широкая электрификация южных губерний даст мощный толчок подъёму сельского хозяйства.
б г д и п т