For the dynamically populated component you have to init it manually. For the MDB5 Select generated inside the modal (as shown in the previous post) you can try this code:

myModal.addEventListener('shown.mdb.modal', () => {
    const selectEl = document.getElementById('mySelect')
    Select.getInstance(selectEl).dispose()   // only if select was generated before the shown
    new mdb.Select(selectEl)   // reinitializing the select component
}

Hope this will help.

Keep coding, Marcin

I am having the same issues with selectboxes, but only when they are populated at runtime by javascript. The line fixes itself away if you click the selectbox, seems like a state issue.