Thank you both for your ideas (this time I have hit the Like button!)
Unfortunately those solutions didn't work. I upped the Top Count limit, activated the pagination, but the same results.
I'm not sure it is related to the Get Items or the List table, because it collect the data in Excel correctely, and get the info from SharePoint correctly as well, because the update is working well. I don't understand why the creation part was working and not anymore.
Here is the code for the whole "Create Item", maybe it helped (I tried to anonymize some part of the code, please let me know if I let something in it that I shouldn't have!)
{"id":"bd57a9d1-8e95-xxxxxxx","brandColor":"#036C70","connectionReferences":{"shared_excelonlinebusiness_1":{"connection":{"id":"/providers/Microsoft.PowerApps/apis/shared_excelonlinebusiness/connections/shared-excelonlinebu-043bae83-xxxxxxx"}},"shared_sharepointonline_1":{"connection":{"id":"/providers/Microsoft.PowerApps/apis/shared_sharepointonline/connections/shared-sharepointonl-4409ad72-2a57-xxxxxxxxxxx"}},"shared_sharepointonline":{"connection":{"id":"/providers/Microsoft.PowerApps/apis/shared_sharepointonline/connections/shared-sharepointonl-d71e3022-71f4-xxxxxxxxxxxxxx"}}},"connectorDisplayName":"SharePoint","icon":"https://connectoricons-prod.azureedge.net/sharepointonline/icon_1.0.1329.1953.png","isTrigger":false,"operationName":"Create_item","operationDefinition":{"type":"ApiConnection","inputs":{"host":{"connection":{"name":"@parameters('$connections')['shared_sharepointonline_1']['connectionId']"}},"method":"post","body":{"Title":"@items('Apply_to_each')?['Project ID']","Project_x0020_Name":"@items('Apply_to_each')?['Project Name']","Project_x0020_Description":"@items('Apply_to_each')?['Project Description']","Programme":"@items('Apply_to_each')?['Programme']","Project_x0020_Status":"@items('Apply_to_each')?['Project Status']","Start":"@items('Apply_to_each')?['Start']","Technical_x0020_Go_x0020_Live":"@items('Apply_to_each')?['Technical Go Live']","Business_x0020_Go_x0020_Live":"@items('Apply_to_each')?['Business Go Live']","Planned_x0020_Closure":"@items('Apply_to_each')?['Planned Closure']","Progress":"@items('Apply_to_each')?['Progress']","Phase":"@items('Apply_to_each')?['Phase']","PM":"@items('Apply_to_each')?['PM']","Lead_x0020_Architect":"@items('Apply_to_each')?['Lead Architect']","OTP_x0020_Number":"@items('Apply_to_each')?['OTP Number']","Budget_x0020_Cost":"@items('Apply_to_each')?['Budget Cost']","Supplier":"@items('Apply_to_each')?['Service Provider']","SharepointSite":"@items('Apply_to_each')?['Sharepoint Project Site']","BRM":"@items('Apply_to_each')?['BRM']","PeerReviewer":"@items('Apply_to_each')?['Peer Reviewer']","ServiceManager":"@items('Apply_to_each')?['Service Manager']","ProjCreatedDate":"@items('Apply_to_each')?['Created']","IAM":"@items('Apply_to_each')?['IAM?']","Dynamics":"@items('Apply_to_each')?['Dynamics?']","OData__x004f_365":"@items('Apply_to_each')?['O365?']","Service_x0020_Now":"@items('Apply_to_each')?['ServiceNow?']","SAP":"@items('Apply_to_each')?['SAP?']","Other":"@items('Apply_to_each')?['Other?']","ProjectDepartment":"@items('Apply_to_each')?['Department']","BudgetOwner":"@items('Apply_to_each')?['Budget owner']","LastSteerCo":"@items('Apply_to_each')?['Last SteerCo']","NextSteerCo":"@items('Apply_to_each')?['Next SteerCo']"},"path":"/datasets/@{encodeURIComponent(encodeURIComponent('https://xxxxxxx'))}/tables/@{encodeURIComponent(encodeURIComponent('2f7b98dc-da35-4f55-9cdc-2c62fbbeba07'))}/items","authentication":"@parameters('$authentication')"},"runAfter":{},"metadata":{"flowSystemMetadata":{"swaggerOperationId":"PostItem"}}}}