Thank you very much for your feedback, @huizijian!
It has been reported to our engineers and they'll check to problem.
Any further questions, please feel free to tell us.

Could you please tell us whether your task is a repeated task?

Sorry for late reply.
The tasks where this ploblem happens ARE NOT REPEATED TASKS.
I'll wait your kind advice.

@huizijian Thank you for your feedback!

If you drag and drop a task to a start date later than the deadline, then the deadline is supposed to be equal to start date.
For example,
Initially: start date = Sep 15, deadline = Sep17
When start date is changed to Sep 16, then: deadline = Sep 17
When start date is changed to Sep 20, then: deadline = Sep 20

We've tested iPhone version, Android version, Windows version, Mac version, web version, and found all work as mentioned above.

Do you come across that problem frequently?
Could you please sign out and sign in again to see if it still exists?
If you encounter the problem again, could you please tell us the task ID: go to the web version, click that task to enter its detail page, then copy its url in the browser address column and send it to us.

Hope it helps.
Look forward to your reply.
Have a nice day!

全ての端末でサインアウトし、再度ログインしましたが、発生します。
発生するのは「今日」から「明日」にドラッグ＆ドロップした場合のみです。

以下の移動の場合は発生しません。
「今日」⇒「スケジュール」
「明日」⇒「今日」
「明日」⇒「スケジュール」
「スケジュール」⇒「今日」
「スケジュール」⇒「明日」

次のコメントにタスクＩＤ添付します。