Problem of scrolling at modal window switc

Hello, I have two modals on my page. One to login and one to register.

In the login modal there is a button "Not registered yet? Register" button that closes the login modal and opens the registration modal.

Same thing, in the registration modal, I have a link "Already registered? Log in" which closes the registration modal and opens the connection modal.

When I open directly the connection or registration modal, the scrolling of the page makes the modal scroll, it's perfect. My HTML parser (Firefox) does put a scrollable tag next to <div class="modal fade show" [...] > and an overflow tag next to <div class="modal-content">.

On the other hand, when I switch from one modal window to another, the scrolling remains attached to the page behind.

I tried to switch modals with HTML attributes or in JavaScript using the toggle() or hide()/show() methods or even by simulating the click on the modal's open button. But nothing helps, the bug persists.

I also compared the HTML rendering of the modal window with and without the bug. Nothing differs.

Do you have an idea?