Sometimes, I've encountered situations where merging two or more objects using the Join function in Object Mode results in the disappearance of the UV map from one or more of the original objects, despite the UV map being clearly visible for each object before the Join, even in the case of multiple object selections...
Video Transcript
Sometimes, I've encountered situations where merging two or more objects using the Join function in Object Mode results in the disappearance of the UV map from one or more of the original objects, despite the UV map being clearly visible for each object before the Join, even in the case of multiple object selections.
Upon closer inspection, it's possible to observe that the UV map of the objects is actually there but reduced to a small dot in the bottom left corner. This outcome typically occurs when the UV layout hasn't been properly unwrapped or errors have occurred during the operation.
There could be various reasons for this peculiar result, but in 99% of cases, it's due to the original objects having different names for their visible UV maps. This situation can easily arise when combining objects from different projects or even different software, as each of them might have a different name for the main UV map.
I've specified visible or main UV maps because an object can have multiple UV maps, for example, for lightmaps in game engines. However, only one of these maps is considered visible. This uniqueness is maintained even when merging multiple objects because all UV maps are retained, but only one can be visible, leading to the issue.
In the example I'm showing on video, there are three objects: an apple, its stem, and an arrow passing through the apple. These are three separate objects sharing the same material and UV layout without overlaps, as confirmed by selecting all objects and entering Edit mode.
Examining the UV Maps section in the Object Data Properties tab individually for each object, we notice that the apple and the stem have two UV maps, but the main one is named uv_set_1, while the second map, called UVMap, lacks a properly sewn layout.
Now, performing the Join, one of the two layouts will no longer be available... but it's still there, residing in the second UV map. I then undo this Join and return to the previous situation with the three objects still separated.
In this particular example, all you need to do is rename the UVMap layout of the arrow to uv_set_1 before proceeding with the Join between the apple and the arrow, which will now yield the desired result.
