Pose jump after source switching OPT_FLOW->EXT_NAV(Tertiary->Secondary) after EXT_NAV reset

## Bug report

**Issue details**

While trying to handle EXT_NAV(VIO) source resets during mission execution by using EKF Source Switching, I notice an abrupt pose jump while switching from OPT_FLOW->EXT_NAV after EXT_NAV source reset in LOITER and GUIDED modes.

**Sources list:**
* Primary: GPS
* Secondary: EXTNAV(VIO)
* Tertiary: OPT_FLOW

**Workflow**
I'm using custom LUA scipt to do manual source switching, trying to mimic the resets and source switching workflow. I manually switched from GPS->EXTNAV before starting the mission. When EXTNAV resets(0,0,0,0,0,0) we switch from EXTNAV->OPT_FLOW. Once EXT_NAV becomes healthy, we switch back from OPT_FLOW->EXTNAV. The aim is to automatically switch between sources when needed using LUA script. Reference: [ahrs-source.lua](https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Scripting/examples/ahrs-source.lua)

Help is much appreciated in understanding this behaviour. @rmackay9 @priseborough

**Version**
4.1.5

**Platform**
[ ] All
[ ] AntennaTracker
[X] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
Quadcopter(IRIS copter)

**Hardware type**
Gazebo SITL Ubuntu 18.04

**Logs**
[log.zip](https://github.com/ArduPilot/ardupilot/files/11650687/log.zip)

**Video**
[video link](https://mega.nz/file/xqMgGBTJ#MoXnW0qDA1VALphnXjfpnDoGXimGTAHN82A_kD_3KT0)

Start by reviewing the linked ahrs-source.lua example and the attached log.zip and video, focusing on the OPT_FLOW→EXTNAV transition after reset in LOITER and GUIDED modes. Trace the source-switching implementation; no specific source file or test is named. Done means the pose jump is reproduced and its cause or a verified correction is documented.

Written by the indexing model from the issue text.