If you apply a stroke to a live text using Acumin Variable Concept, Myriad Variable Concept, Minion Variable Concept fonts, the stroke stays solid.
If you expand these, glyphs are united.

However, if you do the same with Bahnschrift, Cascadia Code, Clash Grotesk Variable, Source Code Variable (and perhaps some other fonts), you get glyphs partitioned.

To workaround this, we can use Pathfinder Add effect... but it won’t help if you have different fills applied to your characters.

Why the results are different? why Illustrator treats some variable fonts as a collection of sub-parts, but others stay wholesome?