I do not have a Yahoo account to test with but do not see the same issue with other email services I use.
Consider trying the following to help troubleshoot the cause:
Sign into Yahoo Mail via there webmail page, star a newly sent message, and sign out
in Vivaldi, press Ctrl + F12 to access Settings, then select Mail and, under Mail Settings, Turn on IMAP Logs in Console
press Ctrl + Shift + J to open Developer Tools Console and select Console tab
in the Filter field type fetch labelvivexample
press Ctrl + Shift + M to open / return to Mail Panel and select the message that was 'starred' in Yahoo Mail
select Label and then Add New Labelsโฆ
type Example and select Save; note new label is automatically added to selected message
press Ctrl + Shift + J to return to Developer Tools Console
copy and post the resulting output shown here after having redacted each line's content up to S:, for example:
S: * 1 FETCH (UID 1 MODSEQ (20) FLAGS (\Flagged \Seen NonJunk $labelvivExample ))
S: * 1 FETCH (UID 108 MODSEQ (280) FLAGS (\Seen $labelvivExample))
S: * 1 FETCH (UID 1 FLAGS (\Flagged \Seen NonJunk $labelvivExample) MODSEQ (20))
S: * 1 FETCH (UID 108 FLAGS (\Seen $labelvivExample) MODSEQ (280))
press
Ctrl + F12, then select
Mail and, under
Mail Settings, deselect
Turn on IMAP Logs in Console
If \Flagged attribute is set then Vivaldi need to look at issue otherwise it is Yahoo