Converting “single line text” to “single select” leads to data loss
It appears that the issue happens whenever there are values which differ only by upper/lowercase of one of the characters.
It also seems that this happens for cells which include multiple words, one of which occurs in other cells in the column (test B vs B, as seen in the screenshots)
there may be other cases. Oddly enough, I have not come across this issue for multi selects - but it may be worth to check it out for other combinations of characters.
This does certainly appear to be the case that it is not automatically applying the related single-select value automatically, but if you copy paste the data again in to that same column after the options have all been converted from single line text, it will apply the appropriate single-select option to it. Not ideal though I agree so I will make a gitlab issue for this.
Well, any copy-paste workarounds apply only to super small tables in practice.
For anything larger, like 10 thousand rows (or more) this actually is a huge problem, especially if one has more select values.