Zero variance of random effects after removing of correlation in linear mixed models

Question

I have some hierarchical data (roughly 23 observations per individual, 20 individuals per region, and 17 regions in total), and use linear mixed models (LMM) to adjust for the dependencies that come due to the hierarchical nature of the data. I thus run the following model in R:

lmer(Y ~ X1 + X2 + X3 + X4 + X5 + X6 + X7 + X8 + X9 + X10 + X11 + X12 + (1 | region) + (1 + X1 + X2 + X3 + X4 | region:ID), data=MyData)

When I estimated my model with four random slopes and a random intercept, using the command I got the following correlations among my random effects:

Random effects:
 Groups      Name                 Variance Std.Dev. Corr                   
 region:ID   (Intercept)          79.53741 8.9184                          
             X1                    0.30512 0.5524    0.48                  
             X2                    0.06766 0.2601   -0.50 -0.68            
             X3                    0.14973 0.3870   -0.57  0.00 -0.40      
             X4                    0.52897 0.7273   -0.79 -0.28 -0.09  0.95
 region      (Intercept)           4.93030 2.2204                          
 Residual                         16.88022 4.1086                          
Number of obs: 9091, groups:  region:ID, 383; region, 17

Noting that the correlation between the random effects of $X_4$ and $X_3$ is high, I tried also ran the model while forcing independence on the random slopes. That is I ran:

lmer(Y ~ X1 + X2 + X3 + X4 + X5 + X6 + X7 + X8 + X9 + X10 + X11 + X12 + (1 | region) + (1 + X1 + X2 + X3 + X4 || region:ID), data=MyData)

However, once I did this, the random effects structure was changed so that variance of the random effects for $X_1$ and $X_2$ suddenly was zero.

Random effects:
 Groups        Name                 Variance Std.Dev.
 region.ID     X1                    0.2246  0.4739  
 region.ID     X2                    0.0000  0.0000  
 region.ID     X3                    0.0000  0.0000  
 region.ID     X4                    0.3236  0.5689  
 region.ID     (Intercept)          11.4392  3.3822  
 region        (Intercept)           9.3745  3.0618  
 Residual                           18.1717  4.2628  
Number of obs: 9091, groups:  region:ID, 383; region, 17

My question is thus, why can the exclusion correlation parameters lead to non-zero variances becoming zero?

Edit:

$X_1,X_2,X_3$ and $X_4$ are all continuous. However, they are measured on the region level. So for a given year (it's longitudinal data) each individual within the same region has the same value of those variables. So they vary between regions and between years, but not within a region-year.