Yes, a lowercase "u" exists for Calibri.
It sounds like you're experiencing an issue with the font on your system. Try resetting the Character panel in After Effects. To do so, choose Window > Character, click the panel pop-up menu and the last option is Reset Character. Create a new Text layer with the default font and a few words that use a lowercase "u" ("The quick brown fox jumps over the lazy dog" is standard for evaluating the characters of a font), select the entire line, and then change the font from the default to Calibri Bold.
If that still doesn't show as expected, try the same text in Photoshop or Illustrator. If that works, try selecting that text and copy and paste it to a text prompt in After Effects. If there's still an issue, remove all instances of Calibri from your computer (be it from Adobe, Monotype, or another source), restart, and then try reinstalling the font with one font manager.
Here is an After Effects 2025 project (25.0.1) that uses Calibri Bold that you can try opening as part of your troubleshooting: https://f.io/YNNsMjaC.
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