I guess it possible that these were just terrible artists, But it seems more likely that they just didn't know what a lion actually looked like. Afterall, if you were an English medieval monk, how likely were you to see an actual lion? They were no photos to go off.

Check out this Cambodian lion: https://artuk.org/discover/artworks/a-guardian-lion-281362 The craftsmanship is quite impressive. But it doesn't look anything much like a lion IMHO.

I guess that Cambodian lion is rather like the Chinese lions, which "are intended to reflect the emotion of the animal as opposed to the reality of the lion" [1], falling into the bucket with intentionally unrealistic (non-lifelike) art.

[1] https://en.wikipedia.org/wiki/Chinese_guardian_lions#Appeara...

There were a few English artists who might have had the opportunity of knowing what they looked like, given there were lions at the Tower of London:

https://en.wikipedia.org/wiki/Tower_of_London

"Records of 1210–1212 show payments to lion keepers" - although who was allowed to visit the lions is a different question ...