Yarn 1.22.4 not accepted

I'm getting this error while running building my app, in package.json I've configured it for yarn 1.22.4 and nodejs 12.18.1, why can't it just install yarn@1.22.4? It works when manually running yarn.
```
[INFO] Installing yarn@1.22.4
--
289 |  
290 | > yarn@1.22.17 preinstall /layers/heroku_nodejs-engine/nodejs/lib/node_modules/yarn
291 | > :; (node ./preinstall.js > /dev/null 2>&1 \|\| true)
292 |  
293 | /layers/heroku_nodejs-engine/nodejs/bin/yarn -> /layers/heroku_nodejs-engine/nodejs/lib/node_modules/yarn/bin/yarn.js
294 | /layers/heroku_nodejs-engine/nodejs/bin/yarnpkg -> /layers/heroku_nodejs-engine/nodejs/lib/node_modules/yarn/bin/yarn.js
295 | + yarn@1.22.17
296 | added 1 package in 0.704s
297 | ---> Installing node modules from ./yarn.lock
298 | yarn install v1.22.17
299 | [1/5] Validating package.json...
300 | error myapp@1.0.0: The engine "yarn" is incompatible with this module. Expected version "1.22.4". Got "1.22.17"
301 | error Found incompatible module.
```

**EDIT**
Solved by using version 0.3.8 of the buildpack.. shouldn't this be part of the detection process though?