Unexpected behavior on re-adding a private tag to a dataset after remove_private_tags

[eeilon79]

I've encountered this issue while trying to write a solution to #1087.
I'm trying to add a past private tag to a dataset that had all his private tags removed with remove_private_tags. It leads to a single element update - just the private tag data. No private creator element is added. Also, the private tag is updated on the second free block rather than the first - that's supposed to be free after remove_private_tags.
It seems the dataset preserves some information on past private creators blocks even after remove_private_tags.

I expect that a private creator element would be (re-)created if I'm trying to add a private tag, and that it would created on the first available place in the private group.

Reproducing code:

from pydicom import Dataset

ds = Dataset()
block = ds.private_block(0x000b, 'dog^1', create=True)
block.add_new(0x01, "SH", "Border Collie")
block = ds.private_block(0x000b, 'dog^2', create=True)
block.add_new(0x01, "SH", "Poodle")

ds.remove_private_tags()
block = ds.private_block(0x000b, 'dog^2', create=True)
block.add_new(0x01, "SH", "Poodle")
print(ds)

Output:

(000b, 1101) Private tag data                    SH: 'Poodle'`

Aa far as I understand, output should be:

(000b, 0010) Private Creator                     LO: 'dog^2'
(000b, 1001) Private tag data                    SH: 'Poodle'

My environment (Running with docker from PyCharm):
Linux-4.15.0-74-generic-x86_64-with-debian-buster-sid
Python 3.6.9 |Anaconda, Inc.| (default, Jul 30 2019, 19:07:31)
[GCC 7.3.0]
pydicom 2.0.0.dev