Welcome to Microsoft Q&A! Thanks for posting the question.
Apologies for any inconvenience with this issue.
I have validated this on multiple test Webapps (as you mentioned), the certificate appears to work fine (kindly check the screenshot below).
If you haven’t done this already, just to isolate, kindly try this on multiple browsers/ you may try clearing cache/cookies and then check to see if that helps.
Having said, I wish to engage with you offline for a closer look (with WebApp and subscription details) and provide a quick and specialized assistance, please send an email with subject line “Attn:Ajay” to AzCommunity[at]Microsoft[dot]com referencing this thread and the Azure subscription ID, I will follow-up with you.
Thanks for your patience and co-operation.