I believe these log messages explain why the upmaps are being removed, but I'll attach the relevant section of the log as well:
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.1->[4,11,12,9,1,10]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.2->[6,11,8,1]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.3->[6,9]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.5->[6,11]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.7->[12,7]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.8->[6,1]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.9->[17,10,8,1]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.b->[0,11,8,4,6,9]
...
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.47->[12,9]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.48->[6,7]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.4d->[2,3]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.4e->[6,17]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.4f->[10,17]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.55->[6,3]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.57->[2,3]
2018-08-27 13:33:25.988 7fa8dee42700 10 maybe_remove_pg_upmaps cancel invalid pg_upmap_items entry 5.6a->[12,3]