Was about to report the same, did some testing in 1.13:
-In my survival world, I have a large map made of many 1:2 maps. All banner markers on these consistently disappear.
-In the same world, I have a map made from 4 1:16 maps. The markers of the same banners that disappeared on the 1:2 maps do NOT disappear here.
-I created a a superflat world for testing. First tried making one map per zoom level of the same area and test it with the same banner, no markers disappeared.
-I then created another banner in the spawn chunk and tested again with the same maps (1:1 excluded, since the chunk was not included). On the 1:16 map this marker (not, however, the first one!) disappeares consistently, on all other maps it stays.
-It does not appear to make a difference wether the map is in an inventory or an item frame when unloading and reloading.
EDIT: Just tested both worlds in the latest snapshot, 18w32a, and was unable to reproduce the error with the maps that previously had had issues.
Björn Seine
Was about to report the same, did some testing in 1.13:
-In my survival world, I have a large map made of many 1:2 maps. All banner markers on these consistently disappear.
-In the same world, I have a map made from 4 1:16 maps. The markers of the same banners that disappeared on the 1:2 maps do NOT disappear here.
-I created a a superflat world for testing. First tried making one map per zoom level of the same area and test it with the same banner, no markers disappeared.
-I then created another banner in the spawn chunk and tested again with the same maps (1:1 excluded, since the chunk was not included). On the 1:16 map this marker (not, however, the first one!) disappeares consistently, on all other maps it stays.
-It does not appear to make a difference wether the map is in an inventory or an item frame when unloading and reloading.
EDIT: Just tested both worlds in the latest snapshot, 18w32a, and was unable to reproduce the error with the maps that previously had had issues.