This bug is not a duplicate of MC-88181
This bug does not affect versions prior to 1.15 unlike MC-88181 (tested in 1.14.4 and 1.12.2)

This bug is related to protocol change in version 19w34a (1.15)
This version removed the Metadata field from the Spawn Packet.

For example, if you summon an entity with an invisibility tag that is applied beforehand (for example, using the Bukkit API), then with some probability you will see this entity for 1 frame, the probability will increase if the client has a high ping or high fps.

I don't see any reason why this was done, since the server sends the Metadata Packet after the Spawn Packet anyway.

Possible solutions to the problem:

• Return metadata field to Spawn Packet
• Make it so that the client does not render the entity until it receives the first packet of metadata about it
• Add boolean invisible field to Spawn Packet (I think it's a bad idea)

Most likely these solutions will at least partially fix bug MC-88181

UPD.
If you decide to fix this, it would also be cool to have an Equipment field in the spawn packet, it will also make life easier for plugin developers