Packages of group configs are merged with global package configs

[Linhtinh603]

Describe the bug
When configuring multiple API groups in Springdoc OpenAPI (2.8.13), the API endpoints from one group are being merged into another group when the global property springdoc.packages-to-scan is defined.

For example:
When accessing /api-docs/external-api-v3, the OpenAPI docs also includes endpoints from external-api-v4.
When accessing /api-docs/external-api-v4, it also includes endpoints from external-api-v3.

This happens only when springdoc.packages-to-scan is set.
If it is left empty, the groups work correctly, but then /api-docs includes all packages (including internal ones), which is not desired.

To Reproduce
Configuration:

springdoc.packages-to-scan=com.my.server.v3.external.api, com.my.server.v4.external.api
springdoc.group-configs[0].group=external-api-v3
springdoc.group-configs[0].display-name=External API v3
springdoc.group-configs[0].packages-to-scan=com.my.server.v3.external.api
springdoc.group-configs[1].group=external-api-v4
springdoc.group-configs[1].display-name=External API v4
springdoc.group-configs[1].packages-to-scan=com.my.server.v4.external.api

Project package structure:

com.my.server.v3.external.api
com.my.server.v4.external.api
com.my.server.v3.internal.api
com.my.server.v4.internal.api

Version information
org.springdoc:springdoc-openapi-starter-webmvc-ui: 2.8.13
Spring Boot: 3.4.8

Expected behavior
With the above configuration:
/api-docs/external-api-v3 → should only contain endpoints from com.my.server.v3.external.api
/api-docs/external-api-v4 → should only contain endpoints from com.my.server.v4.external.api
/api-docs → should include both v3 and v4 external APIs, but not any internal APIs.

Actual behavior
Both /api-docs/external-api-v3 and /api-docs/external-api-v4 include endpoints from both external API packages when springdoc.packages-to-scan is configured.

Workarounds
Setting springdoc.packages-to-scan= (empty) fixes the group merging,
but causes /api-docs to include all packages, including internal ones.

Using springdoc.packages-to-exclude works temporarily,
but it breaks when new packages are added to the project.