Confirmed. Fishman = Loren Fishman. See update below original story.
original March 10 post:
New Mallard Fillmore comic strips are being signed “Fishman.”
After about 3½ months of reruns new Mallard Fillmore strips have begun appearing this week.
But they are not being signed by creator Bruce Tinsley.
Last year Reed Brennan notified subscribers to the comic that, “Due to unforeseen circumstances, reprints are being issued for the MALLARD FILLMORE daily strips for release Nov. 11-16, 2019. A Sunday reprint for Nov. 24 will also be provided.”
After a few notices they stopped the announcements, though the reruns continued.
Now, beginning March 9, 2020, new Mallard Fillmore strips have appeared with the Fishman signature.
No announcement has been forthcoming, and it is unknown as to whether this is a temporary or permanent situation. The general consensus is that the new cartoonist is Loren Fishman,
though that has not been confirmed*. The circumstances involving Bruce Tinsley remain a mystery.
*update March 23:
I can confirm that cartoonist Loren Fishman is “Fishman.” It may be a week or two before King Features publicly acknowledges it, but that recognition is forthcoming.