I can reproduce it locally very easily. Here are the steps:

1. Make sure $wgFlowEnableOptInBetaFeature = false;
2. Create a new user
3. Check "Automatically enable all new beta features" in it's preferences
4. Set $wgFlowEnableOptInBetaFeature = true;
5. Go to Special:Preferences
6. Go to it's talk page -> it is a Flow board

Unfortunately, I don't see a simple way to opt-out of the auto-enroll-all by looking at the BetaFeatures code, and the possibility to opt-out was assumed but doesn't seem to have been confirmed or challenged in the original ticket T98270: Opt-in for Flow on your own user talk page.

Change 242714 had a related patch set uploaded (by Sbisson):
[WIP] Allow preferences to be exempted from auto-enroll

https://gerrit.wikimedia.org/r/242714

Change 242715 had a related patch set uploaded (by Sbisson):
[WIP] Disable auto-enroll for opt-in beta feature

https://gerrit.wikimedia.org/r/242715

Scenario for testing after the fix(the issue was confirmed in betalabs for ET20 user):

1. Find a user that has wikitext talk page but does not have 'Automatically enable all new beta features' enabled.

2.Click ' Automatically enable all new beta features' - go to User Talk to confirm that it's not converted.

Note(helpful query)

SELECT up_user,  GROUP_CONCAT(up_property), up_value  
FROM user_properties  where up_property in ('beta-feature-flow-user-talk-page', 'betafeatures-auto-enroll' )  GROUP BY up_user;

Change 242714 merged by jenkins-bot:
Allow preferences to be exempted from auto-enroll

https://gerrit.wikimedia.org/r/242714

Change 242715 merged by jenkins-bot:
Disable auto-enroll for opt-in beta feature

https://gerrit.wikimedia.org/r/242715

Checked in betalabs - including for possible co-dependency upon opting-out between switches 'Automatically enable all new beta features' and 'Flow on user talk'.

However, the fix does not work retroactively - users who already had Flow User talk page incorrectly enabled due to enabling
'Automatically enable all new beta features' option, cannot return to their previous wikitext User talk page even after disabling 'Automatically enable all new beta features' .

You may check for user ET20 (will try to keep user settings intact): both settings for all Beta features and Flow page are disabled, but User talk page is still Flow-enabled:

[enwiki]> select * from user_properties where up_user=6916;
+---------+----------------------------------+----------+
| up_user | up_property                      | up_value |
+---------+----------------------------------+----------+
|    6916 | beta-feature-flow-user-talk-page | 0        |
|    6916 | cx                               | 0        |
|    6916 | timecorrection                   | Offset|0 |
|    6916 | uls-compact-links                | 0        |
|    6916 | visualeditor-enable              | 1        |
|    6916 | wikibase-otherprojects           | 0        |
+---------+----------------------------------+----------+
6 rows in set (0.00 sec)

[enwiki]>  select page_id, page_title, page_namespace, page_content_model  from page where page_id=129674;
+---------+------------+----------------+--------------------+
| page_id | page_title | page_namespace | page_content_model |
+---------+------------+----------------+--------------------+
|  129674 | ET20       |              3 | flow-board         |
+---------+------------+----------------+--------------------+
1 row in set (0.00 sec)

Note: Would be nice to check for all such discrepancies(in production too).