Do I have to replace any withdrawals back into the same ISA from which they were made?

If you subscribe to a flexible ISA, and later withdraw these subscriptions in the same tax year, then you will be entitled to make replacement subscriptions into any ISA (including ISA's with other providers) as long as you follow the ISA rules and do not subscribe to more than one ISA of the same type within the tax year.

For example, if you have subscribed to a Flexible Cash ISA, and subsequently withdraw these subscriptions in the same tax year, you would only be entitled to make replacement subscriptions up to the amount subscribed in that tax year to a Stocks and Shares ISA, Lifetime ISA or Innovative Finance ISA, but not another Cash ISA as ISA rules prohibit subscribing to more than one of the same type of ISA in any one tax year.