I managed to get the desired effect by inserting flow breaks (???) . . . before the notes (see below)

In the html presentation generated with the revised code

• NOTE 1 is visible after fragment 2, until TEXT 2
• NOTE 2 is visible after fragment 3

Cannot say whether the flow breaks (. . .) are mentioned in the quarto documentation; I found out about them in Meghan Hall's Making Slides in Quarto with reveal.js

---
title: TEST
subtitle: _notes visibility in flow_
self-contained: true
embed-resources: true
engine: knitr
format: revealjs
---

```{=html}

<style>

.reveal .slides section .fragment.step-fade-in-then-out {
    opacity: 0;
    display: none; }
.reveal .slides section .fragment.step-fade-in-then-out.current-fragment {
    opacity: 1;
    display: inline; }

</style>

```

## Slide title {.center}

. . .

TEXT 1
[ fragment 1]{.fragment .fade-in-then-out}
<br>
[fragment 2]{.fragment}


. . .

::: {.notes}
NOTES 1 : should be visible AFTER fragment 2, until TEXT 2
:::

. . .

<br />
TEXT 2

::: incremental

* list 1
* list 2 [&nbsp;- fragment 3]{.fragment}

:::

. . .

::: {.notes}
NOTES 2 : should be visible AFTER fragment 3
:::