user127600: Imagine there are two collision blocks A and B. These are mined at the same time.
One miner created new block C on top of block A and longest chain happened.
Do transactions of block B go back to mempool immediately after being created block C?

user126793: 
Imagine there are two collision blocks A and B. These are mined at the same time.

If both miners are neutral, the set of transactions in A is similar to the set of transactions in B.

One miner created new block C on top of block A and longest chain happened.

If the third miner is neutral too, it is likely that some transactions in B not in A are in C. And some in A's parent block.

Do transactions of block B go back to mempool immediately after being created block C?

See https://bitcoin.stackexchange.com/a/109920/126793, attention at what is commented there: New transactions start as UNCONFIRMED. At BlockConnected, they will transition to CONFIRMED. In case of reorg, at BlockDisconnected, they roll back to UNCONFIRMED..

127600: <p>Imagine there are two collision blocks A and B. These are mined at the same time.
One miner created new block C on top of block A and longest chain happened.</p>
<p>Do transactions of block B go back to mempool immediately after being created block C?</p>
 126793: <blockquote>
<p>Imagine there are two collision blocks A and B. These are mined at the same time.</p>
</blockquote>
<p>If both miners are neutral, the set of transactions in A is similar to the set of transactions in B.</p>
<blockquote>
<p>One miner created new block C on top of block A and longest chain happened.</p>
</blockquote>
<p>If the third miner is neutral too, it is likely that some transactions in B not in A are in C. And some in A's parent block.</p>
<blockquote>
<p>Do transactions of block B go back to mempool immediately after being created block C?</p>
</blockquote>
<p>See <a href="https://bitcoin.stackexchange.com/a/109920/126793">https://bitcoin.stackexchange.com/a/109920/126793</a>, attention at what is commented there: <code>New transactions start as UNCONFIRMED. At BlockConnected, they will transition to CONFIRMED. In case of reorg, at BlockDisconnected, they roll back to UNCONFIRMED.</code>.</p>
