I can duplicate this and have moved the topic to bugs and edited the subject.
This is a result of planned design. If all line items on a transaction have the same unit of measure, the unit name will be substituted as the header for the Qty column. If they have different names, Qty remains and the units of measure are shown on individual lines.
However, that substitution should only occur when all line items have the same unit of measure, not just because only one unit name is present. Line items now included with no unit of measure can be mislabeled with the unit of measure from another line item.
No there isn’t. Please show the edit screen of the Purchase Invoice and that of the Item. I am sure the item will have Rft as unit. This is as @Tut described expected behaviour. The Qty changes to unit when one or more items in the same invoice use that unit. The bug was when someone mixed items with named units with those that had no names and wrongly assigned, for example Rft to such non-named unit. In the instance where there are multiple units inclusive of non named ones then Qty will show, when all items like in your case use the same unit (Rft) then Qty gets replaced with that unit.