Looks like the sheet got a commit 13 days ago to add the advanceX attributes to the skill rolls, without wrapping them in parentheses. For example <select class="charaselect" name="attr_InquiryCharacteristic">
<option value="@{Fellowship}">(Ag)</option>
</select>
Became <select class="charaselect" name="attr_InquiryCharacteristic">
<option value="@{Fellowship} + @{advanceFel}">(Fel)</option>
</select>
The roll button is then using "@{InquiryCharacteristic}/2", which is getting expanded to "@{Fellowship} + @{advanceFel}/2" Olav Müller is responsible for this specific change, although I don't know what their Roll20 account is, since they didn't add their account id to the sheet.json (but they did add their name to the authors field of sheet.json). Andy W. said: why is the modification is being applied retroactively to existing sheets? Because the character sheet system has no concept of versioning. If the campaign creator has a Pro subscription, you could use a "custom" sheet with content from the GitHub repo's history, to go back to a previous version of the sheet.