minecraft: 1.21.1
fabric: 0.16.9
tweakeroo: 0.21.52
timestamp setting does not show first set of colons between the numbers. adding a second colon between each fixes this (displays ONE colon when 2 are in the config)
default config:

output:

working config:

output:

Is this somehow related to some character set or Java version stuff or what... Or some other mod messing with the text field and parsing a single colon as something special. It still works fine for me with just a single colon in the 0.21.53 version (and malilib 0.21.2), same as in 1.20.4 and before. I didn't test 0.21.52 specifically though.