(English is not my mother tongue so that there might be some mistakes in this text)

I used this command to summon a dolphin near conduit in a small space:

/summon minecraft:dolphin 8371.500 6.500 -759.500 {Tags:["This_is_a_tag"],PersistenceRequired:1b,Silent:1b}

However, the dolphin quickly swam to a corner of the space and died.

But if I /summon a dolphin without any modified data values, nothing happened.

But when I tried this under a lake, its AI works ordinary. 

(I have to compress attachments to make it smaller than 10M, so it is not very clear)