Tracks now show up as albums on iPhone Music app -- a sudden change

I assume that the album is a compilation, with each track performed by a different artist. In this case, the Album Artist field has to be filled in with the text Various Artists and the compilation flag needs to be set on.

Select all the tracks on the album and tap iTunes' top bar menu Edit/Get Info. Using the screenshot below, check that the album artist and compilation tags are as shown in the picture:

Note that the artist tag will show Mixed (in italics) as there are different artists for each track.

If there is a dash in the compilation box, that indicates that not all the selected tracks have the same information Tap the box twice: first to turn off the tag and second to turn it back on for all tracks:

Once you have completed this, Sync the phone with iTunes for those changes to be reflected on the phone.