I’ve had this exact issue since last week on version 3.0.0.25175.
In fact, if you look closely, when you click and hold down the Hubspot side panel “sprocket” icon, Calendar is being clicked, simultaneously. 
I create Hubspot support case 16225348739 and supplied them all sorts of logs, videos, HAR recordings and documentation.
Version 3.0.0.25329
Today they said they “fixed” it with version 3.0.0.25329. Not quite, though.
First, now, when clicking the Hubspot “sprocket” icon, it still appears to clicks Calendar, simultaneously (click and hold the Hubspot icon, you’ll see what I mean).
Second, Calendar opens for a fraction of a second (something still very wrong with the extension), before it opens Hubspot - but in full-screen (blocking the view of Gmail entirely), not in the right-hand side panel, as it always has in the past. Opening full-screen, blocking the view of Gmail, defeats the purpose of a side-panel extension.
It’s worth noting that all other side-panel extensions, including third party extensions like Canva, continue to work flawlessly. Only Hubspot has this issue.
Related issue
I’ve also noticed that when the current version of the Hubspot extension is active, when composing new Gmail emails in full-screen mode, the “send” message and other controls are cut off the bottom of the screen, and there are multiple vertical scroll-bars. This never used to be an issue, either (I’ve always composed in full-screen mode and never had an issue until late last week)
Now, composing email with extension ON (note controls are cut off, and there are multiple vertical scroll bars on the right):
Composing email with extension OFF (controls are visible, no vertical scroll bars on the right):
I wish Hubspot would revert the extension to whatever version preceded 3.0.0.25175 - because until then, the Gmail extension worked just fine.