I saw this with older versions, just tried it with claws-mail 3.2.0 and claws-mail-vcallendar 1.100 with removed configuration dir to make sure nothing influences the test.
I set up an IMAP account with local sendmail as sending server, and everything works fine, and my timezone is shown as UTC+1. I load the vCallendar plugin, everything is fine. I create a meeting, and that gets created with proper timezone. But after that any mails sent and any meetings created have timezone set to UTC+0, which for one means that in different applications the meetings will be seen at wrong times.