here are two sets of mesh which looks identical, vert edge face count and even the vert index order are the same. when use “transfer uv maps” from 2 to 1, 1 as active object. mesh 2’s uv is messed up (UV vert split up). vice versa. if duplicate them and use transfer uv maps operator on their respective duplicates, ( after altering their UVs a little bit) the results work as expected. how to make sure mesh 1 and 2 are identical? or in this case it is operator transfer uv maps error? here is the file(http://uploadmb.com/dw.php?id=1604139792&/UV map transfer.blend) and screen grab:
2$\begingroup$ Tested it from your file. Seems to work correctly. $\endgroup$– lemonOct 31, 2020 at 15:14
$\begingroup$ take a closer look at the uv vert position, although the general shape is correct, it seems that each face is rotate a bit resulting not a seamless connect between faces. and many uv vert are not welded. Tested in clean install of 2.82a and various other versions 2,79. 2.90. $\endgroup$– sleepyheadNov 1, 2020 at 4:58
$\begingroup$ Seems to be totally identical here i.stack.imgur.com/Z4F4J.jpg. Transfer UV Map is active to selected. Like all Ctrl+L operations. Not sure this is what you mean in the question. $\endgroup$– lemonNov 1, 2020 at 8:04
$\begingroup$ from your screenshot, will you be able to make top left and bottom left meshes display the same texture using transfer uv maps? or have them all display the same? $\endgroup$– sleepyheadNov 1, 2020 at 9:57
1$\begingroup$ The transfer operates but as it is. And as the meshes are different that gives the result you have. Use mesh data transfer, instead (with both meshes overlapping the closest possible). This one will use a projection cage to correct things (use "projected face interpolated" options). That will give a better result. $\endgroup$– lemonNov 1, 2020 at 12:13
a work around is to snap every vert from mesh A to mesh B via vert index and use data transfer by close proxy. please see this post on how to snap it via vert index: snap objects of same topology together by vertex index
special thanks to @lemon for comment here and provide solution in above post.