Since update 4.5.1 Joe is crashing repeatedly on syntax control
Joe is nearly unusable, must downgrade to the previous version

It seems that the bug is known.
In the following link it seems that next version joe 4.6 fixes crash on syntax highlighting, and we can suppose this the bug I encounter

https://joe-editor.sourceforge.io/NEWS.html

I encounter this bug on ALL MY FREEBSD WORKSTATION, in AMD64 and i386
I have 5 FreeBSD installations..

For testing, just open a big shell script (this doesn't happen on rarely on very small scripts), and crashes mainly occur when syntax control must pass on a the character '"'

As a workaround, whenever I know to have to type a '"', I am forced to first comment a line with a "#", I enter my commands including the '"', and when finished I uncomment the line.
Same thing if I want to modify my script, and if I forget....

Yes... bul..shit 

So in my opninion, it could be wise to downgrade the port to the previous version, and to wait for the version 4.6.
The version 4.5.1 is really not a "decent" version for production.
This is a MAJOR bug which renders the editor nearly unusable.
The bug has not be reported yet because people use probably other editors, but personally JOE is my main editor to write scripts in shell. EMACS is too "heavy" for me.

As an alternative, you could also try to import the coming version if possible and after testing