Does ProtonVPN indeed rely on ISRG Root X1?

[freedom-foundation]

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Proton customer support claims the ProtonVPN app does not rely on ISRG after I reported an ACME Anvil bug. However I have just tested the app again after disabling all SSL certificates in android then reenabling some certificates after each connection attempt by the app hoping to see ProtonVPN connect with it's original SwissSign certificates by enabling swisssign SSL, proton does not connect when SwissSign is enabled, and note that ProtonVPN does not connect until ISRG is enabled. Does ProtonVPN indeed rely on ISRG Root X1?
When I say indeed I ask if this is intentional or an SSL hijacking brought about by confusion.