Fields input screen will not retain colons

Anki Card Design
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Trying several field names with colons in them, I keep seeing no result when I go to the Card screen. Then, when I return to the Fields input screen, the colon has disappeared.
Example: Target: line. The Card fails because Target: line reverts to Target line, with no colon. I’ve put back the colon several times and saved, but the colon always disappears. Are colons no longer acceptable as part of a field name?
This sort of field name has worked previously, though it’s been a couple of years and probably a different version of ANKI.
FWIW: I’m using Windows 10 on a Lenovo and
ANKI Version 2.1.33 (3f403040)

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Thanks a lot!