v1.6.2.3 fix: error in lag formulation

- Following correction proposed #34 (comment 22395)
- Correct tests consequently

Refs #34
3 jobs for sd in 10 minutes and 44 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build_patched #104669

00:00:20

 
  Tests
passed check_as_cran_patched #104671

00:01:27

passed check_not_cran_patched #104670

00:08:55