A previous issue, MC-122088, covered this with regard to storage into scoreboards.

At the time, the bug was closed as "works as intended" as scoreboards are not intended to hold "long" values.

However, the same problem occurs when attempting to store into storage when explicitly setting a type of "long"

eg:

/execute store result storage t:seed aLong long 1 run seed

=> Seed: [5486814582284274048]

/data get storage t:seed aLong

=> Storage t:seed has the following contents: -847459968L