UPDATED:

• Fixed issue with custom model data on 1.21.4.
• Added the option to use the old custom model data for 1.21.4 (will be removed at some point and is only to be used if you have issues with the strings and floats).
  • To use the old Int format add a new line under custommodeldata called old-cmd. Add your custom model data value to BOTH old-cmd and floats.

For example:

custommodeldata:
  strings: []
  floats: ["1234567"]
  old-cmd: 1234567

The default download version is now 1.21.5+. To use 1.21.4, download the version from below.