Trouble upgrading to react-query v5

[Devon-Dickson]

I'm working on upgrading from React Query v4 to v5 and I'm getting these errors when building with Webpack. I understand this might have something to do with Webpack selecting the wrong build files?
Unfortunately, I'm struggling to create a reproduction in stackblitz, but I'm hoping there's an immediately obvious cause that someone can point out.

frontend-1      | ERROR in ../node_modules/@tanstack/query-devtools/build/index.js 5:2
frontend-1      | Module parse failed: Unexpected character '#' (5:2)
frontend-1      | You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
frontend-1      | | // src/TanstackQueryDevtools.tsx
frontend-1      | | var TanstackQueryDevtools = class {
frontend-1      | >   #client;
frontend-1      | |   #onlineManager;
frontend-1      | |   #queryFlavor;
frontend-1      |  @ ../node_modules/@tanstack/react-query-devtools/build/legacy/ReactQueryDevtools.js 6:0-65 20:8-29
frontend-1      |  @ ../node_modules/@tanstack/react-query-devtools/build/legacy/index.js
frontend-1      |  @ ../app/static/frontend/App.jsx
frontend-1      |  @ ../app/static/frontend/routing/index.tsx
frontend-1      |  @ ../app/static/frontend/index.jsx
frontend-1      |  @ multi @babel/polyfill index.jsx

frontend-1      | ERROR in ../node_modules/@tanstack/query-core/build/legacy/queriesObserver.js 116:85
frontend-1      | Module parse failed: Unexpected token (116:85)
frontend-1      | You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
frontend-1      | |       result,
frontend-1      | |       (r) => {
frontend-1      | >         return __privateMethod(this, _combineResult, combineResult_fn).call(this, r ?? result, combine);
frontend-1      | |       },
frontend-1      | |       () => {
frontend-1      |  @ ../node_modules/@tanstack/query-core/build/legacy/index.js 8:0-55 34:0-60:2
frontend-1      |  @ ../node_modules/@tanstack/react-query/build/legacy/index.js
frontend-1      |  @ ../frontend/services/pacClassicTransaction.ts
frontend-1      |  @ ../frontend/pages/pac/transactions/pacTransaction.tsx
frontend-1      |  @ ../app/static/frontend/routing/index.tsx
frontend-1      |  @ ../app/static/frontend/index.jsx
frontend-1      |  @ multi @babel/polyfill index.jsx

etc etc etc

[Devon-Dickson]

Asked the same question in the discord and another user provided this PR that dropped support for Webpack v4. Unfortunate for my situation, but good to know the cause.

#5507