By default (in the absence of a CA-signed server certificate), RDP connections between Windows PCs rely on a trust-on-first-use (TOFU) model, where the client software displays a warning [1] before sending credentials to a server whose certificate is not pinned to the registry [2].
(This is somewhat similar to SSH with password, although the auto-generated self-signed RDP server certificates are only valid for 6 months for some reason, so users eventually learn to just ignore these warnings.)
Microsoft's RDP client for iOS used to work like the Windows client in this respect [3], but over the last few years I have noticed that while the Windows client kept periodically prompting me to acknowledge new self-signed certificates, the iOS client stopped doing so.
Recently, I manually generated self-signed 10-year certificates for my personal laptops in an attempt to fix the TOFU model. Sure enough, while the Windows client warned me about these unfamiliar certificates, the iOS client just kept connecting. Neither reinstalling the app nor using a FQDN to connect had any effect.
I tried contacting MSRC [4], but they just responded with boilerplate ("closed as a non-MSRC case" etc.). Could someone who works for MS tell if the app's silence on iOS is a bug or a feature? (And if it's a feature, why can't it be turned off?)
[1] https://i.sstatic.net/pu5YX.png
[2] HKCU\SOFTWARE\Microsoft\Terminal Server Client\Servers\...\CertHash
[2] https://nextpointhost.com/images/knowledgebase/how_to_access_forex_vps_via_rdc_using_iphone_or_ipad_6.PNG
[3] https://msrc.microsoft.com/report/vulnerability/auth/
