Download and copy the attached php file to your local moodle test instance.

Set $usecustomid = true; at line 10.

Load the page, get the html code and paste it in https://validator.w3.org/nu/#textarea

HTML code is not valid.

Error: Duplicate ID id0_test1.

Error: Duplicate ID id1_test1.

...

Setting $usecustomid = false; at line 10 the issue disappears.

Moodle should change the id of the tag span used for the mform element feedback.

<span class="form-control-feedback invalid-feedback" id="invite" >