QB Issue Resolution:

if when you start an expense for a vendor you see the old expenses there, then you have the setting in company setting set to one, for prefill transactions with previously entered content

set it to off, or delete/overwrite each line on the new expense

Resolution for Issue 'Entering expenses under the same supplier' available: Yes (Solved).
Source: Intuit Community forum.

Entering expenses under the same supplier: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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