[BUG] BackLink field value replacing with link object on save()

[virrius]

Describe the bug
When modifying a parameter and subsequently saving the model using .save(), I noticed an issue where a previously fetched BackLink field reverted its value from the fetched content back to a reference object.
Further use of the object model after saving may lead to errors.

To Reproduce

import asyncio

from beanie import BackLink, Document, Link, init_beanie
from motor.motor_asyncio import AsyncIOMotorClient
from pydantic import Field


class Task(Document):
    subtasks: list[BackLink["SubTask"]] = Field(json_schema_extra={"original_field": "related_task_id"})
    n: int = 0

    class Settings:
        max_nesting_depths_per_field = {
            "subtasks": 1  # Nesting depth for a specific field
        }


class SubTask(Document):
    related_task_id: Link[Task]
    index: int = 0


async def main():
    db = AsyncIOMotorClient(
        'mongodb://admin:[email protected]/?retryWrites=true&w=majority',
        serverSelectionTimeoutMS=5000
    )['beanie_trouble_test']
    await init_beanie(db, document_models=[Task, SubTask])

    # preparing
    task = await Task().save()
    await SubTask(related_task_id=task.id, index=1).save()
    await SubTask(related_task_id=task.id, index=2).save()
    await SubTask(related_task_id=task.id, index=3).save()

    # testing
    task = await Task.get(task.id, fetch_links=True)
    print("fetched task: ", task)

    print("list of subtasks:")
    for sub in task.subtasks:
        print(sub)

    task.n = 100
    await task.save()
    print("task after saving: ", task)
    print("list of subtasks:")
    for sub in task.subtasks:
        print(sub)


if __name__ == '__main__':
    asyncio.run(main())

logs:

fetched task:  id=ObjectId('66c498fb13d341db6da587bc') revision_id=None subtasks=[SubTask(id=ObjectId('66c498fb13d341db6da587bd'), revision_id=None, related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE947610>, index=1), SubTask(id=ObjectId('66c498fb13d341db6da587be'), revision_id=None, related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE947650>, index=2), SubTask(id=ObjectId('66c498fb13d341db6da587bf'), revision_id=None, related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE946890>, index=3)] n=0
list of subtasks:
id=ObjectId('66c498fb13d341db6da587bd') revision_id=None related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE947610> index=1
id=ObjectId('66c498fb13d341db6da587be') revision_id=None related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE947650> index=2
id=ObjectId('66c498fb13d341db6da587bf') revision_id=None related_task_id=<beanie.odm.fields.Link object at 0x000002A7DE946890> index=3
task after saving:  id=ObjectId('66c498fb13d341db6da587bc') revision_id=None subtasks=[<beanie.odm.fields.BackLink object at 0x000002A7DE947210>] n=100
list of subtasks:
<beanie.odm.fields.BackLink object at 0x000002A7DE947210>

Expected behavior
I guess, saving should either keep content unchanged or fetch it fully with updated data
Additional context
beanie==1.26.0
pydantic==2.8.2